7.1 Introduction

In 1966 House described transtemporal surgical access to the cerebellopontine angle known as the translabyrinthine pathway, based on the removal of the entire labyrinthine block, providing a broad approach that allows identification of the facial nerve and less aggression to the brainstem, cerebellum, and vessels. ¹

However, this access approach sacrifices the patient’s auditory and vestibular function.

In 1972, Hitselberger and Pulec proposed the retrolabyrinthine or infralabyrinthine approach, derived from the translabyrinthine approach, initially for the correction of functional problems of the trigeminal nerve and selective vestibular neurectomy and later used for decompression of vascular loops. ²

The first auditory brainstem implant (ABI) surgery was performed in 1979 by Dr. William F. House and Dr. William Hitselberger at the Ear House Institute in Los Angeles, USA, in patients with type 2 neurofibromatosis whose tumor was removed and the electrode placed at the same surgical time. ³

In 2001, Colletti et al used a classic retrosigmoid approach in two children with bilateral severe cochlear malformations and cochlear nerve aplasia for ABI. ⁴

In September 2002, the retrosigmoid surgical approach was then extended by Bento et al for removal of acoustic small size schwannoma and also other tumors from the cerebellopontine angle in patients with hearing residual preservation. ⁵

In 2012, Bento et al described the main landmarks of this route to place the brainstem implant. Since then, it has become a surgical practice. ⁶

In 2013, the same group performed the first ABI in a child with bilateral cochlear nerve agenesis using this approach, and since then it has been the main approach chosen for this surgery to this time. ⁷

In 2014, Bento et al also presented similar results as the ones obtained by translabyrinthine and retrosigmoid approaches for ABI surgery. ⁸

The advantage of retrolabyrinthine technique is the preservation of the labyrinthine block in order to minimize the probable changes in balance in children. Currently, most surgeons are using the retrosigmoid approach in cases of auditory brainstem implantation, either for removal of the tumor (neurofibromatosis type 2) or in other indications such as cochlear nerve agenesis or trauma, ossified cochlea, and ear malformation. ⁹

The presigmoid retrolabyrinthic approach presents as advantage the preservation of the labyrinthine, allowing the patient to maintain a normal vestibular function, and also providesadequate exposure of the bulbar nerves and foramen of Luschka. Other advantages of this route are the minimal retraction of the cerebellum, the small opening of the dura mater, less chance of liquoric fistula and bleeding because of vessel injury, shorter period of postoperative recovery in intensive care unit, and no necessity of drainage dispositive placement during this period.

The retrosigmoid approach offers some disadvantages such as the necessity for retraction of the cerebellum.

7.2 Retrolabyrinthine Surgical Technique

The patient is put under general anesthesia (without the use of muscle relaxing drug because of the monitoring of the cranial nerves VII, IX, X, and XI during the procedure).

The patient lies in supine position, with the head turned so that the involved ear side is up. Subcutaneous infiltration of lidocaine 2% associated with adrenaline in the concentration of 1:100,000 is performed, and an incision is made in the postauricular skin from above the helix of the ear to a point approximately 3 cm above the auricle, starting from the tangent plane to the posterior wall of the external auditory canal and about 5 cm from the postauricular groove to the tip of the mastoid. After postauricular incision, soft tissue is maintained above the periosteal and osteoperiosteal flap pedicle is created using the cortical bone close to the mastoid at the end of surgery ¹⁰ (Fig. 7.1).

A C-shaped skin incision begins slightly superior to the pinna, extends posteriorly from the root of the zygoma to the inion, and ends 1 cm below and slightly anterior to the mastoid tip. The attachments of the sternocleidomastoid and splenius capitus muscles to the mastoid tip is exposed and maintained in position. During mastoidectomy, the groove of the digastric muscle is also exposed as one important anatomical landmark, close proximity to the stylomastoid foramen, from where the vertical segment of facial nerve emerge through.

Using an operating microscope and drill, a canal wall-up mastoidectomy is performed and the posterior bone external auditory canal is thinned but preserved. The incus, and lateral and posterior semicircular canals are identified along with the sigmoid sinus and facial nerve. The descending and mastoid segment of the facial nerve and the three semicircular canals are delineated. The bone is widely removed from the sigmoid venous sinus from the sinodural angle to the jugular bulb. If necessary, an island of bone is retained on the central portion of the sinus to facilitate a retractor placement (Fig. 7.2). The dura of the posterior fossa is exposed and incised between the posterior canal, sigmoid sinus, and jugular bulb inferiorly, and is incised just anterior to the sigmoid sinus. The main goal to achieve the visualization of the lower cranial nerves (IX, X and XI) is by drilling the jugular bulb inferiorly in order to obtain an adequate space between the posterior semicircular canal and the jugular bulb.

In very few cases (less than 10%) when you don’t have a good exposure of the cranial nerves, the surgeon can drill the posterior semicircular canal posterior semicircular canal (PSC) and obliterate the cruras.

After the incision of the dura of the posterior fossa, the subarachnoid space is then opened to release spinal fluid in the cisterna. A small part of the cerebellum is exposed by minimal retraction to show the exiting nerve roots in the cerebellopontine angle (Fig. 7.3).

The cochlear nucleus complex, composed of the ventral and dorsal, is the site for the placement of the electrode. The ventral cochlear nucleus is the main neural impulse transmission nucleus of the VIII pair and its axons the main ascending pathway of the cochlear nerve. Both the ventral and dorsal are not visible during surgery and their location depends on the identification of adjacent anatomical structures. The dorsal nucleus is located superiorly to the lateral recess of the fourth ventricle, while the ventral nucleus is covered by the cerebellar peduncle.

Between the emergence of the facial (VII) and glossopharyngeal (IX) nerves lies the lateral recess or foramen of Luschka, just medially to the IX nerve emergence in the brainstem. The preferred location for placement of the electrode of ABI is the foramen of Luschka, where the ventral cochlear nucleus and lower part of the dorsal nucleus are located, as this region is less susceptible to nonauditory stimuli, such as facial and glossopharyngeal.

The most important landmarks used to identify the cochlear nucleus area is the IX nerve. The cochlear nucleus complex may be prominent as the VIII nerve approaches in direction of the IX nerve. The entry into the fourth ventricle through the Luschka foramen may be identified by the choroid plexus which covers the cerebellar flocculus, the most protruded structure visible from the foramen of Luschka. To approach the fourth ventricle, the arachnoid over the foramen is displaced and cut, the venous and arterial net are also detached and elevated and finally, the flocculus and the choroid plexus retracted. To this end, rostromedial retraction of the cerebellum is necessary. The choroid plexus projecting from the lateral recess (foramen of Luschka) and overlying the cochlear nucleus complex is followed and the entrance to the lateral recess is then found (Fig. 7.4).

The opening of the lateral recess is confirmed by the outflow of cerebrospinal fluid. The cochlear nucleus complex is identified since it bulged in the floor of the lateral recess.

The electrode array is then inserted into the lateral recess with the aid of a small forceps.

The correct position of the electrode is estimated with the aid of the intraoperative electrically evoked auditory brainstem response and neural response telemetry.

The importance of the right positioning of the electrode is to avoid side effects of nonauditory neural stimulation. Electrodes placed in the foramen of Luschka have been shown to be effective in auditory stimulation with minimal side effects, besides being stable because of the limited space to place it (Fig. 7.5).

Intraoperative electromyography and evocate potential are used to lead the surgeon during the electrode positioning procedure by receiving auditory responses at the time of placing it.

At the end, after the placement of the electrode, the dural defect is repaired using a graft of temporalis fascia and fibrin glue. The mastoid cavity is obliterated with a free fat graft taken from the anterior abdominal wall, from the left iliac fossa (Fig. 7.6 and Fig. 7.7).

The osteoperiosteal flap is closed; first the periosteum, followed by the muscular suture with an absorbable surgical thread 3.0 wire, and the skin plane using nonabsorbable 4.0 wire. A compressive dressing is applied over the ear and maintained for 3 days.